The evolution of the number density of large disk galaxies in COSMOS

M. T. Sargent, C. M. Carollo, S. J. Lilly, C. Scarlata, R. Feldmann, P. Kampczyk, A. M. Koekemoer, N. Scoville, J. -P. Kneib, A. Leauthaud, R. Massey, J. Rhodes, L. A. M. Tasca, P. Capak, H. J. McCracken, C. Porciani, A. Renzini, Y. Taniguchi, D. J. Thompson, K. Sheth, for the COSMOS collaboration

Published 2006-09-01Version 1

We study a sample of approximately 16,500 galaxies with I_AB <= 22.5 in the COSMOS field. Structural information on the galaxies is derived by fitting single Sersic models to their two-dimensional surface brightness distributions. We investigate the evolution of the number density of disk galaxies larger than 5 kpc between redshift z~1 and the present epoch. To this end, we use the measurements of the half-light radii to construct, as a function of redshift, the size function of both the total disk galaxy population and of disk galaxies split in four bins of bulge-to-disk ratio. Furthermore, we use a selected sample of roughly 1800 SDSS galaxies to calibrate our results with respect to the local universe. We find that: (i) The number density of disk galaxies with intermediate sizes (r_{1/2}~5-7 kpc) remains nearly constant from z~1 to today. (ii) The number density of the largest disks (r_{1/2}>7 kpc) decreases by a factor of about two out to z~1. (iii) There is a constancy in the number density of large bulgeless disks out to z~1; the deficit of large disks at early epochs seems to arise from a smaller number of bulged disks. Our results indicate that the bulk of the large disk galaxy population has completed its growth by z~1, and support the hypothesis that secular evolution processes produce - or at least add stellar mass to - the bulge components of disk galaxies.